Undesired or pathological hyperactivity in the peripheral nervous system occurs in many disorders and diseases. This undesired or pathologic hyperactivity may result in undesired motor or sensory effects. Additionally, some medical conditions resulting in chronic pain (e.g., neuromas) are characterized by undesired afferent activity in peripheral nerves. Treatment may include blocking the conduction of these pathological or undesirable signals to mitigate the effects of these conditions. However, conventional conduction blocks may have employed conventional fixed electrodes that are not adjustable and that lead to undesired side effects.
Conventional nerve electrodes have produced undesired side effects in nerve blocking and nerve stimulating applications. The undesired side effects may include, for example, tissue damage. The damage may occur, for example, due to the size and orientation of electrode contacts. The damage may also occur, for example, due to the fact that conventional electrodes carry current via electrons while nerve tissues carry current via ions. (See, for example, Krames, Elliot S., Neuromodulation, Academic Press, 072009. 152.)
Uncoordinated or unwanted generation of nerve impulses is a major disabling factor in many medical conditions. For example, uncoordinated motor signals produce spasticity in stroke, cerebral palsy and multiple sclerosis, resulting in the inability to make functional movements. Involuntary motor signals in conditions such as tics and choreas produce incapacitating movements. Undesirable sensory signals can result in peripherally generated pain. Over-activity in the autonomic nervous system can produce conditions such as hyperhydrosis. If these nerve impulses can be interrupted along the peripheral nerves in which they travel, these disabling conditions can be managed or eliminated.
Conventional nerve blocks that prevent the propagation of an action potential through a nerve have been achieved using high frequency alternating current (HFAC), using single phase current (e.g., direct current), and through combinations thereof. HFAC, where the frequency ranges between, for example, 2 kHZ and 50 kHZ, can completely yet reversibly block the motor fibers in a peripheral nerve. In a conventional nerve blocking apparatus, HFAC is typically delivered through one or more conventional electrodes, which has provided sub-optimal results due, at least in part, to the design of the conventional fixed electrodes. Conventional fixed (e.g., non-adjustable) electrodes may have been designed with tradeoffs concerning the conflicting goals of mitigating onset response and maintaining an HFAC block.
A block threshold is defined as the lowest amplitude at which an entire nerve is blocked. The block threshold increases directly with frequency. If the HFAC amplitude is above the block threshold, then the conduction block will be maintained. There is an amplitude region below the block threshold where the HFAC produces a significant and prolonged nerve activity. This activity occurs where the HFAC is approximately 50-70% of the block threshold. Minimizing the block threshold facilitates having an HFAC block with a minimal charge injection to the nerve and with minimal power requirements.
HFAC produces a focal block at the site of the electrode, The block is quickly reversible, usually within one second, and is fast acting, usually appearing in less than a couple of seconds. An HFAC nerve block induces an intense neural volley at the onset of HFAC delivery before block occurs. This is referred to as the onset response. The blocking waveform typically used in mammalian experiments is a 2 kHz-40 kHz sinusoidal or square wave with an amplitude of 3 V-10 V, or 1 mA-10 mA for current-controlled studies, with higher frequencies requiring higher amplitudes to achieve block.
Substantially reducing or eliminating the onset response will facilitate making HFAC block more viable for clinical applications. It is desirable to prevent the onset activity from occurring for both motor and sensory applications, because it is likely to result in extreme pain and activation of an innervated end effector (e.g., a muscle).
The onset response is characterized by two phases that are usually distinct. The first phase, “Phase I,” is always present if neural conduction block is achieved. Phase I is characterized by a brief (e.g., tens of msec) and intense neural volley. This intense neural volley manifests itself as a large muscle twitch in whole nerve/muscle preparations. The second phase of the onset response, “Phase II,” is characterized by repetitive firing in the nerve that occurs after the intense volley of Phase I and can last for tens of seconds. In most cases, the nerve accommodates to the HFAC and the repetitive firing diminishes gradually into quiescence. This activity has been demonstrated in both whole nerve/muscle and single fiber preparations.